Radiation detector with converters

ABSTRACT

A high efficiency radiation detector employs longitudinally extending converter elements receiving longitudinally propagating radiation to produce high-energetic electrons received by detector structures in interstitial spaces. The secondary electron generation in this architecture allows great freedom in selection of converter materials and thickness. A variety of detector mechanisms may be used including ionization-type detectors or scintillation-type detector.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on provisional application 60/299,097 filedJun. 18, 2001, and PCT application PTC/US/02/19154 filed Jun. 17, 2002and entitled “Radiation Detector with Laterally Acting Converters” andclaims the benefit thereof.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND OF THE INVENTION

Highly efficient photon detectors play a major role in countlessapplications in physics, nuclear engineering and medical physics. Innuclear engineering, radioactive waste can be characterized with photondetectors using nondestructive assay techniques (PNDA). In medicalphysics, photon detectors are extensively used for diagnostic x-ray andCT imaging, nuclear medicine, and quite recently, radiation therapy ofcancer.

In radiation therapy of cancer, ever more accurate delivery techniquesspur the need for efficient detectors for million electron volt (MeV)photons in order to allow the imaging of the patient during radiationdelivery. In particular, in Tomotherapy, a detector for MeV photons canbe used for both the CT imaging and for verifying the dose received bythe patients.

Referring now to FIG. 1, an ionization detector 10 may be used for thedetection of radiation in the thousand electron-volt (KeV) range such asis used in conventional diagnostic x-ray and CT. The ionization detector10 employs a set of conductive laminae 12 oriented generally along anaxis 14 of the propagating radiation. The lamina 12 may be spaced apartalong a transverse axis generally parallel to the radiation axis 14 inparallel configuration defining between them detector volumes 16. Thedetector volumes 16 may be filled with a gas having a high atomicnumber, such as xenon, which may be further pressurized to increase thedensity of xenon atoms within the detector volume 16.

An incident KeV x-ray 18 entering the detector volume 16 will have ahigh probability of colliding with a xenon atom (not shown) to createone or more secondary electrons 20 within the detector volume 16. Theseelectrons 20 produce negatively and positively charged ions within thedetector volume 16. The height of the detector volume 16 along theradiation axis 14 may be adjusted so that substantially all KeV x-rays18 entering the detector volume 16 will experience one such collision.

Opposite laminae 12 surrounding the given detector volume 16 are biasedwith a voltage source 21 causing the migration of the ionization chargeto the oriented lamina 12. The current generated by such electron flowis measured by a sensitive ammeter circuit 22, providing an indirectmeasure of the amount of incident KeV radiation 18.

The laminae 12 thus first serve as collector plates for the ionizationdetector 10. They also serve to block oblique KeV radiation 18′scattered by the intervening patient from being imaged thus improvingthe sharpness and clarity of the image. The laminae 12 further serve toprevent migration of the electrons 20 between detector volumes 16 suchas would produce cross talk further blurring the image. The laminae 12are optimized in thickness in the transverse direction consistent withthese roles.

The ionization detector 10 of FIG. 1 would not be expected to beefficient for MeV x-rays which would be expected to pass fully throughany practical thickness of xenon, generating relatively few electrons.

Referring now to FIGS. 2 a and 2 b, more efficient detection of MeVx-rays 24 may be accomplished by the use of a converter plate 26 whichconverts the MeV x-rays into high-energetic charged particles which aresubsequently recorded electronically or photonically. In a firstembodiment of FIG. 2 a, a detector 25 uses a converter plate 26 that isan opaque, high density, high atomic number material, such as lead,placed above detector media 28 to convert each photon of MeV x-rays 24into multiple electrons 20. The detector media 28 may be film, anionization-type detector 10, a scintillation detector or otherwell-known detector types.

A high atomic number and/or high-density material is preferred for theconverter plate 26 because it has a high cross-section for theinteraction of high-energy photons. Generally, however, the height 30 ofthe converter plate 26 is limited to less than that required to fillyabsorb the MeV x-rays 24 correspondingly limiting the conversionefficiency of the detector 25. The reason for this is that increasingthe height 30 to provide for more absorption of MeV x-rays becomesfruitless as additional ejected electrons are balanced by increasedabsorption of electrons within the converter plate 26 itself.

Referring to FIG. 2 b, the limitation imposed by the converter plate 26of detector 25 of FIG. 2 a, may be overcome by using a transparentscintillating converter plate 26′ as shown in FIG. 2 b. Here the MeVx-rays 24 striking the scintillating converter plate 26′ produce photons34 which pass through the transparent scintillating converter plate 26′to be received by light detector 36. The transparent scintillatingconverter plate 26′ may be made thick enough to block a greaterproportion of the MeV x-rays 24 because the mobility of photons withinthe transparent scintillating converter plate 26′ is proportionally muchgreater than the mobility of electrons within the solid converter plate26. Transverse movement of the photons within the transparentscintillating converter plate 26′ may be blocked by opaque elements 38which may, for example, be slices cut into the material of transparentscintillating converter plate 26′ and filled with a light and x-rayblocking material so as to define regular detection areas.

Ideally the scintillating material will have a relatively high atomicnumber and great transparency. Unfortunately, the manufacture oftransparent scintillating converter plate 26′ using such high qualityscintillators is significantly more expensive than the manufacture ofconventional converter plate 26 shown in FIG. 2 a and the efficienciesof such radiation detectors remain modest.

What is needed is a relatively simple, inexpensive, and high efficiencyradiation detector suitable for high-energy radiation.

BRIEF SUMMARY OF THE INVENTION

The present inventors have recognized that the height limitation of theconverter plate, such that avoids reabsorption of electrons, may beovercome by breaking the converter plate into a plurality of axiallyextending converter elements. High-energetic electrons and, depending onthe energy of the incident radiation, other positive and negative chargecarriers, exit the converter material into the detector volumes placedbetween the converter elements. Converter elements may now be ofarbitrary height in the longitudinal direction with electrons generatedboth at the top of the converter and the bottom of converter likewiseliberated only a short distance, through the converter element into thedetector. In this way, the problem of electrons being retained by theconverter as it increases in height is substantially eliminated andconverter height sufficient to convert substantially all MeV x-rays canbe contemplated.

Specifically then, the present invention provides a radiation detectorproviding a plurality of converter laminae oriented to extendsubstantially longitudinally along the propagation axis of the radiationand spaced transversely across the axis to define a plurality of axiallyextending detector volumes. Laminae receive radiation longitudinally andliberate electrons into the detector volumes. Detector structure fordetecting electrons liberated into the detector volumes providessubstantially independent signals.

Thus it is one object to provide a new detector geometry that usesrelatively inexpensive converter materials to provide extremely highconverter efficiencies. The longitudinal thickness of the convertermaterial is no longer limited and may be adjusted to provide forabsorption of a substantially greater proportion of the radiation.

The detection structure may be a scintillator within the detector volumeoptically coupled to a photodetector or may be an ionizing gas or othermaterial coupled to a collecting electrode assembly, the latter of whichmay, in part, be the laminae.

Thus it is another object of the invention to provide a new detectorgeometry suitable for use with a number of detecting mechanisms.

The laminae may be substantially parallel plates or may be tubes withcoaxial wires where the detector volumes are the spaces between thetubes and the wires.

Thus it is another object of the invention to provide for the improveddetector structure offering one-dimensional, two dimensional/areal oreven fully general three-dimensional detector versions.

The tubes may contain a coaxial wire and the detector volume may be thespace between the tube and wire, which are used as part of an ionizationchamber. Or the tube may be filled with a scintillating material.

Thus it is another object of the invention to provide for either anareal scintillation or areal ionization-type detector. It another objectof the invention to allow the use of relatively low qualityscintillation materials, for example, those having low atomic number toproduce a high efficiency detection device.

The longitudinal length of the laminae may be sized to substantiallyblock the radiation and the transverse width of the laminae may be lessthan the average propagation distance of an electron in the material ofthe laminae.

Thus it is another object of the invention to provide for a detectorassembly suitable for use with a wide range of radiation energies andconverter materials.

The laminae may be tipped with respect to the radiation axis so as toincrease the area of the detector over which radiation is intercepted bya lamina

Thus it is another object of the invention to provide the benefitsdescribed above while increasing the efficiency of the detector byimproving the capture of radiation by laminae.

The laminae may be aligned with lines of radius extending from adetector focal point and the radiation source may be positioned so thatthe radiation emanates from a point displaced from the focal point. Thisdisplacement would allow to easily place the detector into the radiationbeam without causing the detector signals to be highly sensitive to theexact position of the detector with respect to the radiation source.

It is yet another object of the invention to allow for the use ofoff-the-shelf KeV x-ray detectors for MeV detection. Defocusing thedetector increases the interception of radiation by a lamina changingthe mechanism of the detector from a standard ionization detector to aconverter/ionization detector of the present invention.

The foregoing objects and advantages may not apply to all embodiments ofthe inventions and are not intended to define the scope of theinvention, for which purpose claims are provided. In the followingdescription, reference is made to the accompanying drawings, which forma part hereof, and in which there is shown by way of illustration, apreferred embodiment of the invention. Such embodiment also does notdefine the scope of the invention and reference must be made thereforeto the claims for this purpose.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a prior art ionization detector forKeV x-rays taken along a plane of radiation propagation, as has beendescribed above in the background of the invention;

FIGS. 2 a and 2 b are cross-sectional views similar to that of FIG. 1but of prior art ionization detectors for MeV x-rays having singletransverse converter elements as have also been described above in thebackground of the invention;

FIG. 3 is a cross-sectional view of a detector of the present inventionhaving multiple longitudinal converter elements generatinghigh-energetic electrons exiting the converter media producingionization charges that may be collected in an ionization-type detector;

FIG. 4 is a cross-sectional view of one embodiment of the detectorassembly of FIG. 3 positioned with respect to a radiation source andpresenting longitudinal but tipped converter elements so as to increasethe area of the radiation beam intercepted by the converter elements;

FIG. 5 is a detailed view of FIG. 4 showing the path of adjacent x-rays,both of which are intercepted by tipped converter elements;

FIG. 6 is a simplified schematic view of two converter elements showingimportant dimensions for the converter elements such as depend on thematerial of the converter elements and their application;

FIG. 7 is a figure similar to that of FIG. 4 showing a conventionalCT-type KeV ionization detector modified for use with MeV x-rays bymovement of the focal point of radiation such as causes ionization byhigh-energetic electrons exiting the converter in preference to theintended ionization by direct radiation;

FIG. 8 is a plot of detector efficiency as a function of angle along thedetector of FIG. 7 showing a drop off of efficiency toward the center ofthe detector in which the detector veins are tipped less with respect tothe incident radiation;

FIG. 9 is fragmentary perspective view of an embodiment of the presentinvention for providing an area detector composed of tubes withconcentric wire conductors as the converter elements;

FIG. 10 is a cross-sectional view through the tube and wire constructionof FIG. 9 showing a further embodiment where the gaseous ionizationmedium is replaced with a solid state semiconductor material; and

FIG. 11 is a figure similar to that of FIG. 10 showing a furtherembodiment where the center wire conductor of the tube is replaced witha scintillating material to transmit light to a photo-detecting device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 3, a detector 40 of the present invention providesfor a series of longitudinally extending converter elements 42 alignedgenerally with the local radiation axis 14 of radiation propagation. Theconverter elements 42 may be, for example, planar vanes or may be rodsor other shapes.

Converter elements 42 are separated from each other in a directiontransverse to the radiation axis 14 to create interconverter volumes 44such as may be filled with an ionizing medium such as a gas including,for example, xenon. The gas may be compressed in a housing (not shown)so as to increase the odds of electron-gas interaction in theinterconverter volumes 44.

MeV x-rays 24 received by the detector 40 strike the converter elements42 to produce high-energetic electrons 46 which proceed into theinterconverter volumes 44. The electrons ionize the gas in theinterconverter volumes 44. Some MeV x-rays 24′ will pass completelythrough interconverter volumes 44 without contacting the converterelements 42 and may produce some ionization. However, in the invention,this ionization will be less than the ionization caused byhigh-energetic electrons 46 exiting the converter.

Adjacent converter elements 42 may be given voltages of oppositepolarity so as to provide a biasing field collecting the ionizationcharges whose flow may be measured using current detector circuitry wellknown in the art ionization detectors.

In this embodiment, the material of the converter element 42 ispreferably a conductive metal so as to support the current flows of theionization, however, the function of collecting charge may be separatedfrom the function of converting x-rays to electrons and non-metallicconverter elements having a conductive coating are also possible.Similarly, in this embodiment, the converter elements 42 are preferablycomposed of a high atomic number and/or high-density material so as toreduce their height and so as to provide efficient reduction ofscattered x-rays like the laminae 12 described with respect to FIG. 1.Nevertheless, it will be recognized that a variety of differentmaterials may be used depending on manufacturing convenience, the energyof the radiation, and the desire for compactness.

Referring now to FIG. 4, a detector array 50 may be created by arranginga number of converter elements 42 along an arc of constant radius abouta focal spot 52. A radiation source is placed at the focal spot 52 to asto create a fan beam of radiation whose local radiation axes 14 arelines of radius from the focal spot 52 to the detector array 50. Theconverter elements 42 extending generally longitudinally with respect tothe local radiation axis 14 but are also slightly tipped with respect tothe local radiation axis 14. Referring also to FIG. 5, this tipping ofthe converter elements 42 increases the area over which the radiationbeam, for example, MeV x-rays 24′ will strike a converter element 42 andnot pass unintercepted through an interconverter volume 44. Preferably,the tipping will be equal to the width of the converter element 42 inthe transverse direction over the height of the converter element in thelongitudinal direction. However, more or less tipping may also be used,including none as will be described below. When the converter elements42 are tipped, the height and width of the converter elements 42 may beadjusted to ensure that a path length 56 of MeV x-rays 24′ through theconverter element 42 is sufficient to ensure probable absorption of theMeV x-rays 24′.

The slopped sides of the converter elements 42 such as produced by thetipping as shown in FIG. 4 need not be monotonic but adjacent converterelements 42 may alternatively have, for example, interdigitatingprojections so as to preserve an interconverter volume 44 but to exposeno direct through path between converter elements 42.

Referring to FIG. 6, the preferred dimensions of the converter elements42 will depend on the radiation energy, the material of the converterelements 42 and the desired resolution of the detector. Generally thecenterline spacing 55 of the converter elements 42 will be determined bythe spatial resolution desired in the resultant detector. The width 54of the converter elements 42 will depend on their material and atradeoff between the spacing 55 between converter elements 42 whichdetermines the width 57 of detector material and the width 54 of theconverter elements 42 which determine the amount of conversion, bothwhich relate to conversion efficiency. Potentially the thickness of theconverter element 42 may be quite small making use of breakthroughs inthe production of so-called nano-wires of extremely small diameter.

Referring now to FIG. 7, a conventional CT ionization-type KeV detector58 such as one manufactured by the General Electric Company for its KeVCT machines may be applied for use with MeV x-rays using the presentinvention's mechanism of generating electrons using the laminae of thedetector as converter elements 42. Absent recognition of the conversionproperties of the laminae, use of such a detector for MeV radiationwould be counter intuitive because of the expected low interaction ofMeV radiation with the inter-laminae gas. This particular detector 58provides in effect an array of 50, 738 converter elements 42 formed fromthe tungsten laminae. Up to 500-volt potential may be applied acrossadjacent converter elements 42 in an alternating configuration. For afan beam detector, the height of the detector may be 3.56 cm and thedetector may be 44 cm long to measure a six MeV beam.

Improved sensitivity may be provided by defocusing the detector 58. Asshown in FIG. 7, an actual focal point 60 is defined by the orientationof the laminae 12 such as divided the ionization chamber into detectorvolumes 16. Focal point 60 maybe displaced typically inward by apredetermined amount 61 from the focal spot 52 of the MeV x-rays thuscausing the x-rays from focal spot 52 to strike the laminae 12 at anangle increasing the absorption of radiation and their liberation ofelectrons. For example, the detector 58 may have a focal point of 103.6cm and be placed 141 cm away from focal spot 52.

Referring to FIG. 8, the centermost lamina 12 in region 64, whichdespite this displacement are essentially aligned with radiation fromthe focal spot 52, exhibit a decreased sensitivity in comparison withthose off center lamina in regions 66 which are receiving radiationdirected against their sides as well as their ends. Edge most laminae 12in regions 68 exhibit decreased sensitivity because of shadowing causedby adjacent laminae 12.

Referring now to FIG. 9, an areal detector 70 may be constructed alongthe principals described above, by using a set of longitudinally alignedtubes 72 having coaxial wires 74. Here the interconverter volumes 44 arethose spaces between the walls of the tubes 72 and the wires 74.Inter-tube regions 75 do not serve for detection in this embodiment butare relatively minor in area.

In this embodiment, the coaxial wires 74 may be given a positive chargeto collect negative charge carriers formed by ionization of gas held inthe interconverter volumes 44 between the wires 74 and the walls of thetubes 72 or vice versa. Here both tubes 72 and wires 74 provide forconversion properties projecting liberated electrons for detection. Itwill be understood that the tubes 72 may be packed to define anarbitrary area and that each tube 72 and coaxial wire 74 defines adetector element.

Referring to FIG. 10, in an alternative embodiment, the space betweenthe wire 74 and tube 72 (converter materials) may be filled with asemi-conductor material such as amorphous selenium 76 (detectormaterial) so as to produce hole-electron pairs which may be collected bythe electrodes formed by the wire 74 and tube 72.

Referring now to FIG. 11, in yet a further embodiment, the wire 74 maybe dispensed with and the tube 72 filled with a scintillator material 80receiving the liberated electrons 46 and emitting a photon 82 fordetection by a solid-state photo detector 84. The use of the structureof tubes 72 limits the necessity that the scintillator material 80 havesignificant conversion properties (of converting radiation to photons)or be highly transparent (as its height may be limited by proper choiceof the converter materials of the tube 72). This allows lower costscintillating material to be used. It will be understood from the abovedescription, that the above described invention employing a generatingand liberating electron mechanism may be used for KeV or lower energyradiation including visible light. Generally, the dimensions of thedetector structures are fully scalable with the energy of the incidentradiation. Higher energy of the incident radiation translates to largerdetector structures (converter material and detection material), andlower energy of the incident radiation translates to smaller detectorstructures. As used herein, converter materials are the materials thatcovert radiation photons to electrons and detector materials arematerials that are used in the detection of the electrons (e.g.ionizable gasses or semiconductors). The lower limit of scalability isonly determined by atomic dimensions. Thus, the converter material canbe of a nanometer scale (nanostructure), e.g., having dimensions (forexample the width of the converter elements) less than 100 nanometers.

The longitudinal converter mechanism also has potential application inthe field of radiation sensitive films where converter structures,possibly in the form of freely dispersed filaments or aligned filamentstructures using electrostatic techniques and the like, may be embeddedin the emulsion of the film itself with liberated electrons interactingwith the silver compounds of the emulsion to produce a highersensitivity in the film than that which would normally be provided bythe film alone.

It is specifically intended that the present invention not be limited tothe embodiments and illustrations contained herein, but that modifiedforms of those embodiments including portions of the embodiments andcombinations of elements of different embodiments also be included ascome within the scope of the following claims. For example, the use ofsemiconductor detectors or scintillation detectors could be used withthe embodiment of FIG. 4.

1. A megavoltage radiation detection system comprising: a radiationsource directing megavoltage radiation along a propagation axis; adetector positioned to receive the radiation along the propagation axis,the detector including a plurality of substantially identical converterelements spaced transversely across the detector to define a pluralityof detector volumes between opposed walls of the converter elements,wherein the opposed walls of the converter elements are angled withrespect to the axis of propagation to increase the area over whichradiation is intercepted by the converter elements, the converterelements receiving radiation and generating high-energetic electronsexiting the converter elements into the detector volumes; a voltagesource biasing opposed converter elements; and a circuit providing ameasure of current flowing between opposed converter elements to collectand detect charged high-energetic particles emitted into the detectorvolumes by the converter elements when the radiation interacts with theconverter elements to provide for substantially independent signals,wherein the opposed walls are tipped according to lines of radiusextending from a detector focal point and wherein the radiation sourceis positioned so that the megavoltage radiation emanates from a pointdisplaced from the focal point, whereby the area of the detector inwhich radiation is intercepted by opposed walls is increased.
 2. Themegavoltage radiation detection system of claim 1 wherein the opposedwalls are angled at a substantially constant amount relative to a localaxis of propagation for a system having detector focal distance offsetfrom a point at the radiation source from which radiation emanates.
 3. Amethod of detecting radiation comprising: (a) providing a plurality ofconverter elements spaced transversely across a detector to define aplurality of detector volumes, the converter elements receivingradiation and generating positively and negatively charged particlesexiting the converter elements into the detector volumes; (b) anglingthe walls of the converter elements with respect to the axis ofradiation propagation to increase the area of the detector over whichradiation is intercepted by the converter elements; (c) applying avoltage across adjacent opposed converter elements and measuring acurrent flowing between the opposed converter elements to detect aseries of substantially independent signals related to different opposedconverter elements; and, (d) generating an image from the substantiallyindependent signals; wherein the converter elements are matched to theradiation, in size, composition, and arrangement and wherein theconverter elements are tipped according to lines of radius extendingfrom a detector focal point and including the step of positioning aradiation source so that the radiation emanates from a point displacedfrom the focal point whereby the area of the detector in which radiationis intercepted by the converter elements is increased.
 4. The method ofclaim 3 wherein the walls of the converter elements are aligned withlines of radius extending from a detector focal point and wherein aradiation source is positioned so that the radiation emanates from apoint displaced from the focal point.